The worm gear mechanisms are transmission mechanisms which include a worm wheel meshing with a driving worm so that torque can be transmitted from the worm via the worm wheel to a load side (i.e., load-side component). In the field of such worm gear mechanisms, various techniques have been developed for reducing undesired backlash (see Japanese Patent Application Laid-open Publication Nos. 2001-355700 and 2002-37100). The worm gear mechanism disclosed in Japanese Patent Application Laid-open Publication No. 2001-355700 is outlined in FIGS. 19A to 19C, and the worm gear mechanism disclosed in Japanese Patent Application Laid-open Publication No. 2002-37100 is outlined in FIGS. 20A and 20B.
FIG. 19A shows the worm gear mechanism 200 connected to an electric motor 201, FIG. 19B is a sectional view of the worm gear mechanism 200, and FIG. 19C shows how a worm 202 and worm wheel 204 mesh with each other in the worm gear mechanism 200.
In the worm gear mechanism 200 of FIG. 19A, the driven worm wheel 204, connected to an output shaft 203, meshes with the driving worm 202 connected via a worm shaft 205 to the electric motor 201. As seen from FIGS. 19A to 19C, the worm wheel 204 includes a hub 206 fixedly mounted on the output shaft 203, and first and second gears 207 and 208 resiliently secured on the outer periphery of the hub 206 via a ring-shaped resilient member 209.
Namely, in the worm wheel 204 meshing with the worm 202, the two (first and second) gears 207 and 208 are separated from each other, via the resilient member 209, along an axial direction of the output rotation shaft 203 in phase-shifted relation to each other, and the resilient member 209 allows the two gears 207 and 208 to be slightly rotated relative to each other in a rotational direction of the worm wheel 204 and then resiliently rotated back to their original relative rotational positions after the rotation. In this worm gear mechanism 200, a tooth 202a of the worm 200 is held at its opposite surfaces by teeth 207a and 208a of the first and second gears 207 and 208, so as to minimize undesired backlash.
FIG. 20A shows the worm gear mechanism 300 disclosed in the 2002-37100 publication which is connected to an electric motor 301, and FIG. 20B is a sectional view of the worm gear mechanism 300.
In the worm gear mechanism 300 of FIG. 20A, a driven worm wheel 304, connected to an output shaft 303, meshes with a driving worm 302 connected via a motor shaft 305 to the electric motor 301. As seen in FIG. 20B, each tooth 311 of the worm wheel 304 has a meshing region 312 (shaded portion in the figure) that meshes with a tooth 302a of the worm 302. The worm wheel 304 has an annular holding groove portion 313 that is formed in one side of the tooth width, i.e. face width, (extending in a left-and-right direction of FIG. 20B) of each tooth 311 and located outwardly of the meshing region 312 of each tooth 311, and a rubber O-ring 321 is fitted in and secured to the annular groove portion 313. The rubber O-ring 321 is slightly deformed by contacting a top land (i.e., tooth top surface) 302b of the worm 302, and its resilient restoring force imparts a preload to meshing tooth regions to thereby reduce backlash.
However, in the conventional worm gear mechanism 200 shown in FIGS. 19A to 19C, where the worm wheel 204 comprises the two gears 207 and 208 separated from each other along the axial direction of the rotation shaft, the area of contact, with the tooth 202a of the worm 202, of each tooth of the worm wheel 204 is less than one-half the contact area in the traditional worm wheel having an integral (non-divided) gear. When the worm 202 is rotated in a forward direction, the torque is transmitted from the worm 202 to the teeth 207a of the first gear 207, while, when the worm 202 is rotated in a reverse direction, the torque is transmitted to the teeth 208a of the second gear 208. Particularly, the area where the first and second gears 207 and 208 are separated from each other (i.e., the middle portion of the width of the worm wheel in the illustrated example of FIG. 19B) is just where contact pressure, against the worm 202, of the worm wheel 304 becomes greatest. Therefore, further consideration or improvement has to be made for enhanced durability, wear resistance in particular, of the worm gear mechanism 200.
Further, in the conventional worm gear mechanism 300 of FIGS. 20A and 20B, where the annular holding groove portion 313 is formed in one side of the face width of each tooth 311 and located outwardly of the meshing region 312 of each tooth 311, bending rigidity in a tooth-thickness direction would differ between the opposite sides of each face width, which would make the contact pressure uneven between the opposite sides of the face width. Therefore, in this worm gear mechanism 300 too, further consideration or improvement has to be made for enhanced durability of the worm gear mechanism 300.
Besides, in the worm gear mechanism 300, considerable frictional force is produced by the rubber O-ring 321 held in rubbing contact with the top land 302b of the rotating worm 302. In addition, the worm wheel 304 has a relatively great radius from its rotation center (or rotation axis) to the rubbing contact surface of the rubber O-ring 321. Therefore, there would be produced a great friction torque. It is preferable that such a great friction torque be minimized in order to enhance a torque transmitting efficiency of the worm gear mechanism 300. Further, because the rubber O-ring 321 rubs the top land 302b with a great frequency, further consideration or improvement has to be made for enhanced durability of the O-ring 321.
If the worm gear mechanism 200 of FIGS. 19A to 19C or the worm gear mechanism 300 of FIGS. 20A and 20B is employed in an electric power steering apparatus, it is also required to minimize impinging or hitting sound that would be produced between the teeth as the steering wheel is operated by a vehicle driver, so as to minimize noise sound in a vehicle compartment.
Furthermore, because, as well known, the electric power steering apparatus is constructed to add steering assist torque of an electric motor to a steering system via the worm gear mechanism 200 or 300, it is highly preferable to eliminate the backlash in order to achieve an enhanced steering feel; this is due to the fact that, when the steering wheel is turned back by the driver after being turned in a given direction, presence of the backlash would undesirably delay the steering assist torque transmission from the worm gear mechanism 200 or 300 to the steering system.
For the foregoing reasons, there has been a demand for a technique which can reduce hitting sound between the teeth of the worm and worm wheel, which can enhance durability of the worm gear mechanism and which can maintain appropriate meshing between the worm and the worm wheel.